I'm actually a little disappointed at how little science has been bandied about. All talk about morals, scant discussion of ethics, and biology essentially ignored. Here are the underlying issues from what I can tell:

- Procreation therefrom causes a greater risk of subsequent health complications.
- In a closed system, this statistical downside could theoretically be outweighed by the need to perpetuate.
- Humans are highly social, and sex plays a social role that is not purely biological.
- Many of the factors that would promote sexual activity between two otherwise non-related individuals may also be present in related individuals due to shared upbringing.
- Humans are primed socially to seek homogeneity and homeostasis, but sexual to seek that which is different.

So here goes.

Brother/sister incest carries a known penalty for genetic diversity. The risk of congenital defects increases exponentially. As such, we are likely wired to avoid incest on some level. Certainly, societies that eschew such behavior would suffer fewer defects, increasing their overall survival and success.

However, in cases of extreme isolation, i.e. the only two individuals on an island, the drive to survive may increase, and the power of social taboos would naturally decrease. So as the balance changes, a built-in override mechanism kicks in and incest would ensue.

Like gibbons, humans are highly social animals who use sex socially for non-reproductive purposes, ranging from bonding to dominance. As such, as purely reproductive intercourse decreases relative to more socially-instigated forms, social factors take a greater role in selection of sexual partners. If one chooses partners based upon trust, love, or shared experiences and values rather than the need to successfully procreate, then siblings could get put on the list of potential partners.

Perhaps the most interesting interplay isn't the one between society and biology, or congenital disease versus immediate survival, but rather the balance between social homogeneity versus procreative diversity. (What's that?)

On some fundamental level, humans are wired to seek sexual partners that promote genetic diversity. Historically, it took the form of orgies and forced intercourse with conquered peoples. In a modern setting, the drive is still there, but has been domesticated somewhat. "Kink" and "deviance" are modern ways in which physical attraction to new and strange "opportunities" exhibits itself. Cuckoldry, infidelity, and the masculine imperative for "strange" may be morally discouraged, but are examples of an ongoing genetic push for genetic blending. And "forbidden" becomes the new "wanted."

Meanwhile, social structures are geared toward promoting same-ness. We're wired to tell whether someone is from the same tribe or not. We naturally tend to categorize people as "us" and "them." We tend to gravitate toward someone who is socially similar for friendships and other alliances.

So while it is morally frowned upon, the circumstances are ripe for sibling incest, as siblings will tend to share upbringings, common experiences and traumas, cultural backgrounds, trust, social circles, financial circumstances, and other factors. Sexually, they may not be strange enough to provoke a novelty response, but the very taboo of incest may be enough to do so.

In short, now that the biological pressures against sexual sameness can be mitigated by contraceptive techniques, the two primary forces will be social bonding and sexual strangeness promoted by taboos.

It's really only a matter of time before you're biologically enjoined to go f--- yourself. ;)

I don't think the sense of smell is given enough credit as a human motivator. I think that, more than any other sense, it is tied directly to emotion centers. I think that there are some molecular structures in the scent that tell the receiver to turn off sexual responses regarding this individual (a sibling) - all subconscious and subliminal, of course. Going across this interaction could well stir feeling of revulsion - stronger in some than in others.

That sounds like a lot of conjecture. I understand the concept of pheromones, but I've never heard of them being tied to incest before. Besides, pheromones do not a moral or ethical argument make, so they have nothing to do with the question I asked.

Pure conjecture. But I think it does relate to the question. It could explain why what might seem to be a harmless act (after dealing with the problems with offspring), is pretty universally considered to be morally wrong. Isn't a proposition the first step in a scientific study? Or do you just want a yes/no answer as to morality?

I can understand it at a child level but really - as an adult i think that I would have problems relating to my brother correctly. Its not the sort of adult secret I would want to have with a sibling ... ewww . just no

I used to pick on my little sister all the time. My mom would tell us to quit flirting. The thing is, being raised with a person of the opposite sex exposes you to marriage fallacies from an early age. For example, my sister, while cute, passes gas and bleeds out of her crotch. As a result, I have little capacity for intimacy with her. Maybe a hug on christmas.